1. Field of the Invention
The present invention relates to a method for starting single phase induction motor and an electronic relay using the same, and more particularly, to a method for starting single phase induction motor and an electronic relay using the same, which model a start torque curve with an induced voltage to perform control in a programming scheme, and thus can minimize a start failure and increase compatibility in order for be applied to motors based on various standards.
2. Description of the Related Art
In a single phase induction motor, generally, a rotor magnetic field does not occur because a stator winding has a single phase, but once a rotor begins rotating, an induction torque occurs and thereby the rotor rotates at a synch speed according to the standing wave thereof. Accordingly, the single phase induction motor requires a start method for obtaining a start torque because the equilibrium state of a magnetic field is changed into a disequilibrium state at an initial state.
The single phase induction motor is classified into a split-phase motor, a capacitor motor and a shading-pole motor based on the start method for obtaining the start torque. The split-phase motor uses a start scheme that connects a start winding to a run winding in parallel and occurs a phase difference based on the impedance difference between the start winding and the run winding, thereby starting. The capacitor motor is a motor that obtains a start torque greater than that of the split-phase motor by inserting a capacitor into a start winding in series. The phase of a supply current is shifted by the capacitor that is inserted into the start winding and the supply current having the shifted phase flows through the start winding, and thus the equilibrium of an electromagnetic force is disrupted, thereby obtaining the start torque. Subsequently, when a rotor starts to rotate and an angular velocity increases to the certain number of rotation times, the capacitor is separated by a centrifugal switch and thereby the capacitor motor runs normally.
However, the mechanical centrifugal switch is vulnerable to vibration, and its characteristic is degraded due to mechanical/electrical abrasion that is caused by the occurrence of an arc when switching is frequently performed. Accordingly, an electronic relay is gradually used. An electronic relay for a single phase induction motor serially connects a power semiconductor switching device such as triac to a start winding and controls the semiconductor switching device through a control circuit, thereby enabling a current to flow in the start winding only upon start.
A related art electronic relay simply determines a start completion time with only the level of a voltage (which is induced to a start winding) to perform control, and thus compatibility decreases and a start failure rate is high because characteristic is changed according to the standards and disposition conditions of motors. In the related art electronic relay, moreover, because a control circuit for controlling the gate of a triac is configured with an analog circuit, the efficiency of an internal power source circuit decreases when requiring a large amount of current is consumed and the gate current of a relatively high capacity is required.